1. Field of the Invention
The present invention relates to a thermal flowmeter for measuring a fluid flow rate, the thermal flowmeter including a heat resistor formed on a substrate.
2. Description of the Related Art
The mainstream of a flow rate sensor for detecting an intake air amount of an internal combustion engine of an automobile or the like is, for example, a thermal air mass flow sensor capable of directly measuring a mass flow rate.
Recently, a thermal flowmeter has been proposed which has a sensor element fabricated on a semiconductor substrate of silicon (Si) or the like by use of micromachining technology. In such a semiconductor-type thermal flowmeter, a heat resistor is formed on a thin film portion of several microns formed on the semiconductor substrate.
Since the heat resistor is formed on a thin film, the semiconductor-type thermal flowmeter has a small heat capacity, and hence is capable of rapid response and low-power drive. Moreover, microfabrication makes it easier to form a configuration capable of detecting a temperature variation between the upstream and downstream of the heat resistor, and of thereby judging whether the flow is a forward flow or a reverse flow.
Examples of the thermal flowmeter of the above type are disclosed in Japanese Patent Application Laid-open Publication Nos. 2004-205498 and 2005-283381.
Japanese Patent Application Laid-open Publication No. 2004-205498 discloses a technique which improves sensitivity while reducing power consumption by using boron-doped single crystal silicon to form upstream and downstream heaters, and also by setting the line widths of the heaters at 7 micrometers or greater.
Japanese Patent Application Laid-open Publication No. 2005-283381 discloses a technique for enabling a power supply from an ECU (engine control unit) as a power source for the thermal flowmeter, by reducing the amount of heat produced by a temperature sensing resistor in order that a sensor output error due to variations in the voltage outputted from the ECU can be reduced.
A pressure sensor and a temperature sensor used in an automobile are operable with power supply from an ECU. Thus, if the thermal flowmeter can receive power supply from an ECU rather than from a battery, the thermal flowmeter does not need circuitry for protection against electrical noise and overvoltage as well as a power supply line from a battery.
The techniques disclosed in Japanese Patent Application Laid-open Publication Nos. 2004-205498 and 2005-283381 enable reduction in power consumption by the thermal flowmeter and power supply from an ECU to a thermal flowmeter.